Add serde feature for faster-hex

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This PR want to close #25

We can use faster_hex with serde now, just add #[serde(with = "faster_hex")] above the field of the struct, like:

#[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
struct Simple {
    #[serde(with = "faster_hex")]
    bar: Vec<u8>,
}

There are 6 mod:

1. faster_hex::withpfx_ignorecase
   // Serialize with 0x-prefixed and lower case result. When deserializing, expect 0x-prefixed and don't care case
2. faster_hex::nopfx_ignorecase
   // Serialize without 0x-prefixed and lower case result. When deserializing, expect no 0x-prefixed and don't care case
3. faster_hex::withpfx_lowercase
   // Serialize with 0x-prefixed and lower case result. When deserializing, expect 0x-prefixed and lower case
4. faster_hex::nopfx_lowercase
   // Serialize without 0x-prefixed and lower case result. When deserializing, expect no 0x-prefixed and lower case
5. faster_hex::withpfx_uppercase
   // Serialize with 0x-prefixed and upper case result. When deserializing, expect 0x-prefixed and upper case
6. faster_hex::nopfx_uppercase
   // Serialize without 0x-prefixed and lower case result. When deserializing, expect no 0x-prefixed and upper case

Note: #[serde(with = "faster_hex")] is equal to #[serde(with = "faster_hex::withpfx_ignorecase")]

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I think this PR is ready for review.

Cc. @zhangsoledad @quake